Almost all business applications have horizontal menus and ribbons that take up a decent percentage of a landscape monitor instead of utilising the “spare” screen space on the left or right, and a taskbar usually sits at the bottom or top of the screen eating up even more space (yes I know this can be changed but it’s not the default).
Documents are traditionally printed/read in portrait which is reflected on digital documents.
Programmers often rotate their screens to be portrait in order to see more of the code.
Most web pages rarely seem to make use of horizontal real estate, and scrolling is almost universally vertical. Even phones are utilised in portrait for the vast majority of time, and many web pages are designed for mobile first.
Beyond media consumption and production, it feels like the most commonly used workplace productivity apps are less useful in landscape mode. So why aren’t more office-based computer screens giant squares instead of horizontal rectangles?
Humans (and most other animals) see better side-to-side than up-down. Your eyes are spaced horizontally, giving us a wider horizontal field of vision. People generally prefer putting things side-to-side in work environments, maybe also reflecting how much easier it is to move and work within a horizontal plane than a vertical one. So the upper threshold for monitor width would be longer than the upper threshold for monitor height.
That being said, I know reading is best done in narrower columns, to reduce the amount of left-right movement your eyes need to do which can cause you to lose your place when skimming lines. Three columns of text on a 16:9 monitor is way more readable than one column of text that spans the entire monitor.
And then why do we make an exception for phones which are predominantly used in portrait mode? I guess maybe just for easier 1-handed use? Maybe also to give us more peripheral vision of potential hazards and other things happening in the background when using them, since they’re mobile devices.